1. Field of the Invention
The present invention relates to a developing device for an electrophotographic apparatus which utilizes a one-component toner developing method.
2. Description of the Related Art
An electrostatic type electrophotographic apparatus for use, for example, in a copier, facsimile equipment or laser printer apparatus has a photosensitive drum with a photosensitive layer formed thereon. A charger, exposure device, developing device, transfer device and fixing device are located along and around the photosensitive drum.
As the photosensitive drum is rotated, a charge is formed by the charger on the surface (photosensitive layer) of the photosensitive drum and a toner is supplied by the developing device to the surface of the photosensitive drum to form a toner image on the drum surface.
Then a charge opposite in polarity to the charge on the toner image is applied to a recording paper sheet by the transfer device to allow the toner image on the drum surface to be transferred to the recording sheet under an electrostatic force. Finally, the toner image on the recording sheet is fixed by the fixing device.
Further, a cleaning device has to be provided so as to continuously perform a recording process. The toner remaining on the drum surface after transfer is removed by the cleaning device. The charge remaining on the drum surface is removed by the discharger, followed by a charging step in a continuous cycle.
Some developing devices to be mounted in an electrophotographic apparatus utilize a one-component toner developing method. In this type of device, a non-magnetic toner not containing any carrier is pressed by a toner restricting member against the surface of a toner carrier whereby the toner is formed as a thin layer on the carrier surface, while being triboelectrically charged.
FIG. 40 shows this type of conventional developing device.
This developing device is located parallel to a photosensitive drum 9 and has a casing 1, agitator 2, toner feed roller 3, collecting blade 5 and toner restricting member 6.
The casing 1 has substantially the same length as that of the photosensitive drum and defines an inner space where a one-component toner T is stored. The casing 1 has an opening at which the developing roller 4 is located. The casing 1 is arranged parallel to the photosensitive drum 9.
The toner feed roller 3 and developing roller 4 have substantially the same length as that of the photosensitive drum 9 and are arranged parallel to the photosensitive drum 9 such that they are rotatably supported at the casing 1 side. The developing roller 4 is placed in contact with the outer peripheral surface of the drum 9, noting that the developing roller 4 is formed of electroconductive rubber and fixed to the outer periphery of a metal shaft 4a.
The toner restricting member 6 is located above the developing roller 4 in a manner to be parallel to the photosensitive drum 9. The toner restricting member 6 comprises a leaf spring 7 made of metal and a blade contact unit 8 made of an elastic material, such as rubber. The leaf spring and blade contact unit 8 have substantially the same length as that of the developing roller 4. The blade contact unit 8 is fixed to one side edge of the leaf spring 7 along its length. The other side edge of the leaf spring 7 is fixed to the casing 1. The blade contact 8 is placed, under an elastic force of the leaf spring 7, in contact with the outer periphery of the developing roller 4.
The operation of the developing device is as follows:
A nonmagnetic one-component toner T in a toner storage area of the casing 1 is fed to the toner feed roller 3 by the rotation of the agitator 2. The toner feed roller 3 and developing roller 4 are rotated by a rotation device, not shown. The toner is fed by the toner feed roller 3 onto the outer peripheral surface of the developing roller 4. The blade contact unit 8 of the toner restricting member 6 presses the toner T which is deposited on the peripheral surface of the developing roller 4 against the outer periphery of the developing roller 4. By so doing, the blade contact unit 8 forms the toner T as a thin layer on the outer periphery of the developing roller 4 and, at the same time, the toner is triboelectrically charged. The charged toner T is deposited onto the outer periphery of the drum 9 to allow an electrostatic image which is formed on the outer periphery of the drum 9 to be developed into a visible image.
Such a conventional developing device has drawbacks as will be set forth below.
The toner restricting member 6 is of such a type that the contact unit 8 is fixed directly to the leaf spring 7. The leaf spring 7 performs a double function of holding the blade contact unit 8 in place and bringing the blade contact unit 8 in contact with the developing roller 4 by its spring force. However, the force by which the leaf spring 7 directly acts upon the blade contact unit 8 varies, thus varying the state in which the blade contact unit 8 makes contact with the developing roller 4.
That is, the leaf spring 7 has substantially the same length as that of the developing roller 4 and, because it is provided in a restricted space, is slender in configuration. For this reason, the spring constant is set to a large value so as to obtain a predetermined spring force. If, however, the spring constant of the leaf spring 7 is set to a large value, then the pressure force with which the blade contact unit 8 is pressed by the leaf spring 7 against the developing roller 4 varies greatly in accordance with the wear of the blade contact unit 8 caused by frictional contact with the developing roller 4.
Further, the leaf spring 7, being longer in its lateral direction, is liable to be deformed in its longitudinal direction, thus causing a local variation of the pressure force with which the blade contact unit 8 is pressed by the leaf spring 7 against the developing roller 4.
The force with and position in which the contact unit 8 is contacted by the developing roller 4 depend heavily upon the thickness of the toner layer on the developing roller 4 and amount of toner charged on the roller. The spring force of the leaf spring 7, being largely varied, causes a larger variation in the thickness of the toner layer on the developing roller 4 and in the amount of toner charged thereon. Therefore, an image of poor quality is formed on the recording sheet, thus failing to obtain a better-quality image.
Further, the spring 7 is manufactured by a discharge machining method and hence involves more manufacturing cost.